July 1, 2014

Ancient Arctic Sharks Lived In Brackish Water 50 Million Years Ago

Most sharks today are strictly saltwater fish, however, a new study led by the University of Colorado Boulder and the University of Chicago reveals that this was not the case 50 million years ago. Sharks in the Arctic Ocean during this time lived in brackish water, with approximately the same amount of freshwater found in modern day Lake Ponchatrain in Louisiana.

The study, led by University of Chicago postdoctoral researcher Sora Kim, suggests that the Eocene Arctic sand tiger shark thrived in the brackish water of the western Arctic Ocean. In contrast with the Eocene Arctic sand tiger shark (part of the lamniform group of sharks that includes today's great white, thresher and mako sharks), the modern sand shark in the Atlantic Ocean requires three times the salinity and is very intolerant of low salt levels.

"This study shows the Arctic Ocean was very brackish and had reduced salinity back then," said Kim in a university statement. "The ancient sand tiger sharks that lived in the Arctic during the Eocene were very different than sand tiger sharks living in the Atlantic Ocean today."

The research team, which includes scientists from the University of Wyoming and Delaware State University, believe their findings have important implications for the fate of today's sharks in a warming Arctic region. The Arctic is warming at approximately twice the rate of the rest of the planet, according to CU-Boulder geological sciences Associate Professor Jaelyn Eberle, because of increasing greenhouse gases. This warming has the potential to cause changes in freshwater runoff and atmospheric water vapor, as well as a decrease in salinity that will have consequences for marine biology and seawater circulation dynamics. Their findings were published in a recent issue of Geology.

"As more freshwater flows into the Arctic Ocean due to global warming, I think we are going to see it become more brackish," said Eberle, also curator of fossil vertebrates at the University of Colorado Museum of Natural History. "Maybe the fossil record can shed some light on how the groups of sharks that are with us today may fare in a warming world."

Part of the team's findings on Eocene salinity conditions were calculated by comparing ratios of oxygen isotopes recovered from ancient shark teeth collected from sediments on Banks Island in the Arctic Circle. This data was incorporated into a salinity model and then compared to previous research conducted on sediment cores from the Lomonosov Ridge — a steep hump of continental crust rising more than 1,000 feet from the ocean floor in the central Arctic Ocean. These comparisons allowed the team to estimate past environmental conditions in the Arctic Ocean.

"Oxygen isotopes in ancient bones and teeth reflect the water animals are living in or drinking," said Kim, a former postdoctoral researcher at the University of Wyoming. "Because sharks are aquatic, the oxygen from the ocean is constantly being exchanged with oxygen in their body water, and that's what is incorporated into their teeth. When I analyzed their isotopic composition, the numbers seemed weird at first because they indicated an essentially freshwater environment."

Thirty Eocene sand tiger shark fossilized teeth were collected from Banks Island and analyzed alongside 19 modern sand tiger shark teeth from specimens in Delaware Bay. They found that the paleo-salinity of the modern shark teeth matched the continental shelf salinity from Delaware to Florida, stretching from the coastline to approximately six miles out to sea. This is a known hunting ground for modern sand tiger sharks.

During the Eocene Epoch (56 to 34 million years ago), wild temperature fluctuations and intense periods of greenhouse gases were typical. Lush tropical rainforests covered the Arctic during the Eocene. Previous studies confirmed the existence of the ancestors of tapirs, hippo-like creatures, crocodiles and giant tortoises in this region. Warm humid summers and mild winters, which ranged in temperature from just above freezing to 70 degrees Fahrenheit, dominated the terrestrial Arctic climate despite six months of darkness each year.

"We now know a fair amount about the terrestrial animals and plants that were living in the Eocene Arctic greenhouse period," said Eberle. "To finally get some data on the Eocene marine environment using these shark teeth will help us to begin filling in the gaps."

According to Eberle, the Eocene Arctic Ocean was isolated from other global oceans, for the most part. "Increased freshwater runoff from the land due to an intensified hydrologic cycle and a humid Arctic would have turned it more brackish pretty quickly," she said.

The team found that the Eocene Epoch salinity gradient in the Arctic Ocean was much larger than the salinity gradient tolerated by modern sand tiger sharks living in the Atlantic. "The Eocene lamniform group of sharks had a much broader environmental window than lamniform sharks do today."

Climate researchers use the early-middle Eocene greenhouse period from 53 to 38 million years ago as a deep-time analog for what could occur if the carbon dioxide and other greenhouse gases in our atmosphere continue to rise, and what a potential "runaway" greenhouse effect might look like.

"Through an analysis of fossil sand tiger shark teeth from the western Arctic Ocean, this study offers new evidence for a less salty Arctic Ocean during an ancient 'greenhouse period,'" says Yusheng "Chris" Liu, program director in the National Science Foundation's (NSF) Division of Earth Sciences, which co-funded the research with NSF's Division of Polar Programs. "The results also confirm that the Arctic Ocean was isolated during that long-ago time."

Image 2 (below): CU-Boulder associate professor Jaelyn Eberle, left, and research colleagues collect ancient sharks teeth on Banks Island in the Arctic Circle. Oxygen isotopes in the teeth indicated sharks living in the Eocene Arctic Ocean roughly 50 million years ago were tolerant of brackish water, unlike their shark relatives living today. Credit: Jaelyn Eberle, University of Colorado